This invention relates to packaged solid state devices comprising an encapsulant. The invention also relates to a method for encapsulating a solid state device, such as a light emitting diode (LED).
Solid state devices, sometimes referred to as semiconductor devices or opto-electronic devices, comprise LEDs, CCDs, LSIs, photodiodes, phototransistors, photocouplers, opto-electronic couplers and the like. Such devices often exhibit special packaging needs. High-efficiency, high lumen, solid-state white LEDs require a novel packaging material which can withstand more demanding conditions than those required by typical low-intensity, longer wavelength LEDs. Common packaging materials will often undergo a gradual loss of optical and mechanical properties due to the combination of thermal, oxidative and photodegradation processes.
Resins for encapsulation of solid state devices have primarily relied on blends of bisphenol-A epoxy resins and aliphatic anhydride curing agents. As described in U.S. Pat. No. 4,178,274, to Denk et al., one disadvantage of these compositions, which harden fast through the use of known accelerators such as tertiary amines, imidazoles or boron trifluoride complexes, is their poor thermal aging stability. The materials used heretofore become discolored in extended storage at temperatures above 80° C. The resulting resins, which become yellow to brown, have considerably reduced light transmittancy. Furthermore, because of the aromatic character of bisphenol-A based epoxy resins, these encapsulants are typically less stable to ultraviolet radiation. Therefore, these materials may tend to degrade on extended exposure to light having an ultraviolet component. Such degradation can lead to discoloration of the encapsulant and reduced light transmittance.
To circumvent these issues, Denk et al. describe resin compositions for the sealing of onto-electronic components. These resins comprise a (i) cycloaliphatic epoxy resin, (ii) a carbonic acid anhydride (iii) zinc octoate and (iv) a solvent selected from the group consisting of a low molecular weight polyol, a low molecular weight ester and mixtures thereof. The compositions in Denk et al. are at most 46% epoxy resin by weight. Such low levels of epoxy resin and concomitant high levels of curing agents can lead to color formation in the cured resin, reducing the overall transmittance of a LED.
Wada et al. in U.S. Pat. No. 5,145,889 describe a composition consisting essentially of (i) 100 parts by weight of an epoxy resin (ii) 70 to 140 parts by weight of a curing agent including an acid anhydride (iii) 0.5 to 4.0 parts by weight of a curing accelerator including an onium or diazabicycloalkene salt (iv) 0.5 to 5.0 parts by weight of a phosphorus triphosphite and (v) 0.5 to 5.0 parts by weight of a silane coupling agent represented certain formulas. The compositions in Wada et al. are at most 58% epoxy resin by weight. Such high levels of curing agents can lead to color formation during thermal curing of the resin encapsulant, reducing the overall transmittance of a LED. Furthermore, said encapsulating resin requires the use of a cure accelerator such as an onium or diazabicycloalkene salts to enhance cure rates and allow for reasonable processing times.
Uram in U.S. Pat. No. 4,454,201 describes transparent composite compositions for applications, such as windows for military helicopters, which would require transparency, resistance to heat, abrasion and to penetration by projectiles. A transparent layer in the composite comprises a blend of (i) from about 80 to about 100 parts by weight of an epoxy resin (ii) from about 5 to about 30 parts by weight of a boroxine, such as trimethoxyboroxine (iii) from about 1 part to about 40 parts by weight of an organic phosphorus compound having a specific structure. Whereas this patent teaches boroxine cured epoxy resins in composite applications for military use, encapsulation of opto-electronic devices is not described. Further the benefits of non-aromatic epoxy resins in combination with boroxine curatives for encapsulation of onto-electronic devices is not taught.
Morris in U.S. Pat. No. 4,336,367 describes an adhesive composition for attachment of ordnance devices to the bottom of ships. This composition comprises a mixture of a (i) 3,4-epoxycyclohexylalkyl-3,4-epoxycyclohexanecarboxylate, exo-exo bis(2,3-epoxycyclopentyl)ether, endo-exo bis(2,3-epoxycyclopentyl)ether and (ii) a trialkoxyboroxine. Whereas these epoxy compositions are cycloaliphatic in nature, they are developed for adhesive applications and the use of said compositions for encapsulation of onto-electronic devices was not envisioned.
In copending, commonly owned application Ser. No. 09/654,830, filed Sep. 1, 2000, there is disclosed a composition for the packaging of opto-electronic devices which comprises 1) an epoxy resin and 2) a boron containing catalyst essentially free of halogen. Whereas these materials have excellent optical properties, the thermal properties and moisture absorption characteristics were not optimal for certain onto-electronic applications.
There is a continuing need for novel packaging material for solid state devices, such packaging material desirably possessing properties such as high transmission in a range from near UV to the visible wavelength; long term thermal, oxidative and UV stability; thermal compliance with other materials used to envelope the solid state device; low color; and high reflective index.